H₂O₂ is an important industrial oxidant that can be used in a wide variety of applications, ranging anywhere from chemical synthesis to environmental remediation. Its only reaction by-products, H₂O and O₂, are environmentally benign, presenting a notable advantage over other commercial oxidants. Because of its low molecular weight, it also possesses the greatest fraction of active oxygen by weight percent, making it highly efficient as well as “green”. Currently, H₂O₂ is almost exclusively produced via the anthraquinone oxidation (AO) process. This process requires a high energy input and relies on H₂ from fossil fuels, high pressures, large volumes of organic solvents, and palladium catalysts, ultimately resulting in large amounts of wastewater, exhaust gas, and solid waste. A promising alternative to this production method is the direct reduction of oxygen. The oxygen reduction reaction (ORR) can proceed via two possible paths: a four-electron reaction that results in H₂O, and a two-electron reaction that produces H₂O₂. The two-electron path of the ORR presents a unique opportunity to produce H₂O₂ in a process that is environmentally friendly.
In this work, we explore an n-type organic mixed ionic-electronic conductor (OMIEC) as a catalyst for this two-electron ORR. OMIECs are extremely versatile and can be used in a number of applications, including batteries, super capacitors, electrochemical transistors, electrocatalysts, and more. These materials also present a number of advantages. Most prominently, they are easy to process, made from geo-abundant elements, have a great degree of tunability with respect to structure and physical properties, can achieve volumetric redox activity, and are potentially recyclable.
The OMIEC used in this work is a random copolymer comprised of naphthalene diimidie (NDI) and bithiophene (T2) units, where 75% of the NDI units had glycolated (polar) sidechains. We call this polymer p(NDI-T2 P75). Through the use of rotating ring disk electrode measurements, we find that this OMIEC is capable of catalyzing the ORR and that it is highly selective for the two-electron reaction. Further, using operando UV-Vis and Raman spectroscopy measurements, we find the mechanisms through which this OMIEC becomes catalytically active. To gain further insights about the catalytic reaction path, we construct and test a microkinetic model. Through this model, we show that this material undergoes catalysis through an ECE' reaction path.

William Chueh

Appearances